The Flaviviridae are a group of positive single-stranded RNA viruses with a genome size from 9-15 kb. The Flaviviridae consist of various genera including: Hepaciviruses (this genus contains only one species, the Hepatitis C virus (HCV), which is composed of many genotypes and subtypes); Flaviviruses (this genus includes the Dengue virus, Japanese Tick-Borne and the Yellow Fever virus and there are some additional Flaviviruses that are unclassified) and Pestiviruses (this genus includes three serotypes of bovine viral diarrhoea virus, but no known human pathogens).
Hepatitis C virus (HCV) is a major cause of viral hepatitis and has infected more than 200 million people worldwide. Hepatitis C virus has a positive-strand RNA genome enclosed in a nucleocapsid and lipid envelope. The HCV genome is approximately 9.6 kb in length and encodes a polyprotein of about 3,000 amino acids. There are at least six major genotypes, which have different geographic distributions. In the United States (US), for example, genotypes 1a and 1b account for about 75% of cases, and genotypes 2 and 3 for 10-20% of cases. Significant differences are observed in the geographic distribution of HCV genotypes. For example, in Europe genotypes 2 and 3 comprise up to one half of cases whereas genotype 3 is thought to dominate in India. In addition, varied genotype distributions can be observed between countries in a particular region as well as in different areas of a given nation. In the US, HCV is the most common chronic bloodborne infection, affecting approximately 3.2 million persons. After infection with HCV, approximately 75-85% of people develop chronic infection, whilst 60-70% develop chronic liver disease. Of these, 5-20% go on to develop cirrhosis over a period of 20-30 years, and, finally, 1-5% succumb to the consequences of chronic infection (liver cancer/cirrhosis).
Until recently, the only treatment option for HCV was 24 or 48 weeks of combination therapy consisting of weekly injections of pegylated interferon (peg-IFN) and oral ribavirin for 24 or 48 weeks. The best treatment response is seen in patients with HCV genotypes 2 and 3, in whom sustained viral response (SVR) rates of approximately 80% can be achieved with 24 weeks of therapy. Patients with HCV genotype 1 remain the most difficult to treat, with SVR rates of approximately 40% after 48 weeks of therapy. In addition to the low response rates, combination peg-IFN/ribavirin therapy is limited by serious side effects, including fatigue, influenza-like symptoms, depression and suicide with peg-IFN, and haemolytic anaemia with ribavirin. Furthermore, peg-IFN/ribavirin therapy is contra-indicated in patients who have depression, anaemia, HCV-related decompensated cirrhosis, alcohol/substance abuse and autoimmune disorders or who are pregnant.
New treatment options for HCV became available in May 2011 with the US launch of the first direct-acting antiviral (DAA) HCV drugs, telaprevir (Vertex Pharmaceuticals) and boceprevir (Merck). Both drugs are protease inhibitors and are approved for the treatment of chronic HCV genotype 1 infection in combination with peg-IFN and ribavirin. Pivotal phase 3 trials demonstrated that the addition of telaprevir or boceprevir to peg-IFN/RBV therapy achieved shortened durations of therapy and potent viral suppression, with SVR rates approaching 75% in genotype 1 treatment-naive patients and 30% to 85% in treatment-experienced patients.
However, addition of a third drug to the treatment regimen has resulted in increased adverse events. Telaprevir is associated with an increased incidence of rash and anaemia, while boceprevir is associated with anaemia and dysgeusia. Triple therapy with telaprevir or boceprevir and peg-IFN/ribavirin remains unsuitable for those intolerant to or with contraindications to peg-IFN/ribavirin therapy.
Due to the limited tolerability, efficacy, side effects, genotype coverage and concern over the emergence of resistance there is an ongoing need to find alternative agents for the treatment of HCV. The majority of compounds that are currently in development have a limited spectrum of activity against the various HCV genotypes and, in many cases, are only active against HCV genotypes 1 b and/or 1a.
The HCV genome possesses structural (core) and non-structural (NS2, NS3, NS4A, NS4B, NS5A and NS5B) proteins. The non-structural proteins are involved in viral genomic replication, with the initial synthesis of RNA carried out by NS5B RNA dependent RNA polymerase. The NS5B protein is a key target for anti-HCV therapy, as it is essential for HCV replication and has no human host equivalent. This protein has been well characterised and is a validated target for drug discovery.
HCV therapy is also anticipated to evolve towards oral multidrug therapy, in which combinations of different DAA drugs with complementary mechanisms of action serve to increase viral suppression and delay or prevent the emergence of resistance.
Accordingly, there remains on ongoing need for HCV agents, particularly with targeted mechanisms of action such as NS5B inhibitors. There is also an unmet need for HCV agents with cross-genotypic activity against genotypes 1, 2 and 3.